Opening a large file presents a blank screen for a long time

[vlovich]

If your issue is a build or runtime error, please include the following:

• OS / platform (e.g. macOS 10.13.2)
• the frontend you're using, if applicable (xi-mac)
• the commit you're on, if building from source (#3a2405b) 89d1223

Generate a large random file: dd if=/dev/random of=test.bin bs=1M count=100
Convert to text: hexdump test.bin > test.txt
Open test.txt

Expected:
The view is populated quickly with the contents of the file

Actual:
The view stays blank while the file is loading for a while
100% CPU usage by syntect plugin (why?)
100% CPU usage by xi-core after file loads.

[cmyr]

This has some overlap with #197, Although it does seem slower than it needs to be even in the sync case. Might be fun to investigate once we have the tracing stuff merged.